The present invention relates to a fuel supply system for a direct injection engine in which fuel is injected directly into cylinders of the engine.
A known fuel supply system of an engine having two cylinder arrays, such as a V-type or Horizontal Opposed Type engine, as disclosed in JP-A11-62775, comprises:
a first and a second delivery pipes for distributing fuel (hereinafter also referred to as fuel rails) connected to fuel injection valves (hereinafter referred to as injector) of cylinders in the respective cylinder arrays;
a fuel pump connected to the first delivery pipe at an upstream end thereof;
a connecting pipe connecting the downstream end of the first delivery pipe and the upstream end of the second delivery pipe;
a regulator (means for regulating the fuel pressure) connected to the downstream end of the second delivery pipe; and
a pressure wave transmission means interposed between the upstream end of the first delivery pipe and the downstream end of the second delivery pipe for transmitting pressure waves therebetween.
The pressure wave transmission means is used to reduce the amplitudes of pressure waves which propagate through a fuel supply line and reflected waves as well, and relieve a pressure difference between the far-most parts of the upstream end of the first delivery pipe and the downstream end of the second delivery pipe, thereby reducing uneven fuel injections (in amount) between the cylinder arrays.
In recent years, a fuel injection system has been in the spotlight, by which fuel is injected directly into cylinders of an automobile engine for improving a mileage. The fuel injection system is provided with a fuel pump for further pressurizing fuel in order to directly inject fuel into cylinders under a high pressure.
The known fuel pump mentioned above has a fixed capacity to discharge an excessive amount of fuel than that actually injected from injectors and so constructed that superfluous fuel is returned by the regulator back to the low pressure side of the fuel rail thereby maintaining the pressure in the fuel rails at a substantially constant level.
A fuel supply system having such a fixed-capacity fuel pump disadvantageously lowers the fuel economy of the engine due to the fact that it imposes an extra work on the pump. Consequently, from the viewpoint of fuel economy, a fuel pump having a variable capacity mechanism is preferred. In the fuel supply system with a variable-capacity fuel pump, since the fuel pump is capable of discharging an exact amount of fuel needed by injectors, it may cut wasteful fuel injection and hence wasteful energy consumption. In addition, the variable-capacity fuel pump can advantageously maintain a substantially constant pressure in the fuel rails without utilization of a regulator.
With regard to the fuel supply system having the fixed-capacity fuel pump, since it has the regulator and a return pipe, it is inapplicable or improper to apply to a return-less piping arrangement without a regulator and a return pipe when the variable-capacity fuel pump is used.
Incidentally, the variable-capacity fuel pump is preferably of a single cylinder plunger type since a variable-capacity mechanism can be constructed simply so as to be of a low manufacturing cost. However, it should be noted that the single cylinder plunger pump entails appreciable pressure pulsation in the discharged fuel because the pump operates intermittently with regard to suction and discharge of fuel.
In general, since the fuel pump is driven by the rotational power of engine, the pressure of fuel discharged from the pump pulsates in the fuel rails in synchronism with the engine. On the other hand the injectors connected to the fuel rails are adapted to inject fuel synchronously with the engine. Hence, in order to ensure fuel injections to be in phase with the pressure of the fuel discharged, it is desirable to synchronize the injectors with the pump. This is the case particularly when a single single cylinder plunger pump is utilized, because it generates relatively large fuel pressure pulsation and a little difference in phase between fuel injection and fuel discharge has a significant influence on the amount of the fuel injected by the injectors. It is therefore necessary to adjust the injection timing so as to be in phase with the pump discharge such that the pump discharges fuel once for every two fuel injections by the injectors for two cylinders.
Ideally and preferably, the pump discharges once for every injection by one injector for perfect synchronization. This implies, however, that, given a maximum rotational speed of 8000 rpm for a V-type six-cylinders engine for example, the number of cycles of the reciprocal plunger must be 400 cycles per second, which seems difficult to attain in view of its durability, dimensional limitation, efficiency and other factors of the pump. The difficulty will be amplified for V-type eight and V-type ten cylinders engines.
For these reasons, an acceptable pumping rate for the plunger pump to provide well balanced fueling without suffering from an overload is one time discharge from the pump for injections to two cylinders. This timing, however, has a drawback that the injectors are divided into two groups one of which performs injections during suction strokes of the pump and the other of which performs injections during discharge strokes of the pump, resulting in uneven injections of fuel between the two injector groups.
Furthermore, in V-type six engines for example, because the injection order between two cylinder arrays is alternate, the injectors are completely distinguished to two groups, one of which performs injections during suction strokes of the pump and the other of which performs injections during discharge strokes of the pump, between two fuel rails resulting in enhanced uneven fuel injection therebetween.
Thus, an object of the invention is to provide a fuel supply system for use with direct injection engine, which can reduce the pressure pulsation in fuel rails thereby reducing a variance of fuel injections among engine cylinders.
According to a first aspect of the invention, there is provided a fuel supply system for an engine having a first and a second opposed cylinder arrays each equipped with a fuel injection valve (injector) for injecting fuel thereinto, wherein the fuel supply system comprises:
a low pressure pump for transferring fuel from a fuel tank;
a fuel pump which pressurizes fuel fed from the low pressure pump and comprises a single plunger operating to reciprocate synchronously with the engine;
a first and a second fuel rails provided in association with the first and the second cylinder arrays, respectively;
a pipe line which leads off from the fuel pump, is diverged to two pipe lines and connected to the first and the second fuel rails; and
a first and a second orifices interposed in the pipe line.
According to a second aspect of the invention, there is provided a fuel supply system for an engine having a first and a second opposed cylinder arrays each equipped with a fuel injection valve (injector) for injecting fuel thereinto, wherein the fuel supply system comprises:
a low pressure pump for transferring fuel from a fuel tank;
a fuel pump which pressurizes fuel fed from the low pressure pump and comprises a single plunger operating to reciprocate synchronously with the engine;
a first and a second fuel rails provided in association with the first and the second cylinder arrays, respectively;
a pipe line which leads off from the fuel pump, is diverged to two pipe lines and connected to the first and the second fuel rails;
a first and a second orifices interposed in the pipe line; and
a connecting pipe for interconnecting the first and the second fuel rails with each other at the opposite side ends of the fuel rails to the fuel pump.
Preferably the high pressure pump has a drive mechanism which comprises a rotational power source rotating at one half the rotational speed of the engine shaft like as a camshaft of the engine and a cam for converting the rotational power into the reciprocal motion of the plunger, wherein one rotational motion of the camshaft causes the plunger to reciprocate by one half the number of the combustion cylinders.